1. Field of the Invention
The present invention relates to a transformer, and more specifically, to a transformer formed between two layout layers.
2. Description of the Prior Art
As wireless communication progresses, passive devices such as inductors and transformers are widely used in circuit design for wireless communication. In the field of circuit design, a transformer can be a discrete device or integrated into a chip, however, the high power consumption of a chip may lower the quality level and increase the circuit design difficulties.
Please refer to FIG. 1 which is a layout diagram illustrating a conventional planar transformer 10 according to the prior art. As shown in FIG. 1, the planar transformer 10 includes a primary coil 12 and a secondary coil 14 both on a plane. The primary coil 12 and the secondary coil 14 include two differential signal ends P+, P− and S+, S− respectively. The primary coil 12 spirals inwards around point O from the end P+ to form a necessary number of loops, and then exits the plane from the end P−. The secondary coil 14 spirals inwards around point O from the end S+ to form a necessary number of loops, and then exits the plane from the end S−. The primary coil 12 and the secondary coil 14 are formed on a plane, spiraling alternately to keep from overlapping with each other. It is a disadvantage of the conventional planar transformer 10 that it needs a large-size layout to achieve a necessary inductance. Additionally, the conventional transformer 10 has a poor mutual inductance performance and is not easily applied to circuit design for wireless communication.
Please refer to FIG. 2. FIG. 2 is a layout diagram of a conventional two-level transformer 16 according to the prior art. In order to reduce layout area, as shown in FIG. 2, a two-level conductive coil is used to form the transformer 16. The transformer 16 includes a primary coil 18 and a secondary coil 20. The primary coil 18 and the secondary coil 20 include two differential signal ends P+, P− and S+, S− respectively. The primary coil 18 spirals inwards around point O from the end P+ to form a necessary number of loops, and then ends at the end P−. The secondary coil 20 spirals inwards around point O from the end S+ to form a necessary number of loops, and then ends at the end S−. The primary coil 18 and the secondary coil 20 are formed separately on two different planes. Please refer to FIG. 3 which is an equivalent circuit diagram of a transformer 16. The impedance conversion between differential signal ends P+, P− and S+, S− is determined by the ratio of the number of loops in the primary coil 18 to the number of loops in the secondary coil 20. Although the conventional two-level transformer 16 reduces chip area and increase mutual inductance between the two conductive coils when compared with the conventional planar transformer 10, since the two-level transformer 16 is not symmetrically formed, common mode noise cannot be reduced effectively.
As mentioned above, the conventional planar transformer 10 requires a larger layout area and increases cost. Additionally, a longer conductive coil causes a higher resistance, therefore it results in the inductor consuming more power and lowers the quality level. Although the two-level transformer 16 is capable of reducing layout area, it is not able to reduce common mode noise.